Heterosexual transmission of HTLV-III/LAV, the virus that causes AIDS, has been well documented and may represent the major route of virus spread into the general population. The extent of the problem and the factors associated with heterosexual transmission of HTLV-III/LAV are not well known. The purpose of the current proposal is to determine the prevalence and seroconversion rate of HTLV-III/LAV infection in spouses and heterosexual partners of persons with HTLV-III/LAV will be explored. Spouses and heterosexual partners of index cases newly diagnosed as having HTLV-III/LAV infection will be enrolled. Evaluation will include a medical history, physical examination, an interview, and laboratory tests including T-cells and antibody to HTLV-III. Spouses/partners found to be seronegative for HTLV-III will be followed every 4 months for 3 years. Rates of seroconversion during the study in spouses/partners will be calculated for the entire group and within several subgroups. Whether the virus is more efficiently transmitted from women to men or from men to women will also be evaluated. Of particular importance will be the evaluation of risk factors and mechanisms associated with heterosexual transmission of HTLV-III/LAV. Potential risk factors will be assessed by comparing those spouses/partners who are seronegative for HTLV-III to those who are seropositive. Factors to be evaluated will include socioeconomic status, living conditions, life style, and sexual practices including length of contact, number of sexual encounters, types of sexual activity, and in particular, use of barrier contraceptives. Although HTLV-III/LAV has been isolated from secretions such as saliva and tears, transmission other than through sexual or blood contact has not been documented. Children of index cases with seronegative spouses/partner will be identified and followed. Children will be grouped according to the status of HTLV-III/LAV infection in their parents, those of a seropositive father and seronegative mother and those of a seronegative father and seropositive mother. Finally, if seropositive children are identified, follow-up of their siblings who are seronegative for HTLV-III/LAV will provide another opportunity to evaluate possible household or close contact spread of HTLV-III/LAV.